1. The Field of the Invention
The present invention relates generally to x-ray tubes that use a rotating anode target. More particularly, embodiments of the present invention relate to an improved rotating anode drive assembly, and methods for manufacturing an anode drive assembly, that provide improved mechanical stability in the presence of high operating temperatures.
2. The Relevant Technology
X-ray producing devices are extremely valuable tools that are used in a wide variety of applications, both industrial and medical. For example, such equipment is commonly used in areas such as diagnostic and therapeutic radiology; semiconductor manufacture and fabrication; and materials testing.
The basic premise underlying the production of x-rays in such equipment is very similar. X-rays, or x-radiation, are produced when electrons are released and accelerated, and then stopped abruptly. Typically, the process takes place within an evacuated x-ray tube, which ordinarily includes three primary elements: a cathode, which is the source of electrons; an anode, which is axially spaced apart from the cathode and oriented so as to receive electrons emitted by the cathode; and an electrical circuit for applying a high voltage between the cathode and the anode.
The anode and cathode elements are positioned within the evacuated housing, and then electrically connected. During operation, an electrical current is supplied to the cathode filament, which causes electrons to be emitted. A voltage generation element is then used to apply a very high voltage (ranging from about ten thousand to in excess of hundreds of thousands of volts) between the anode (positive) and the cathode (negative). The high voltage differential causes the emitted electrons to accelerate towards an x-ray xe2x80x9ctargetxe2x80x9d surface positioned on the anode. Preferably, the electron beam is focused at the cathode so that the electrons strike the target surface (sometimes referred to as the focal track) at a defined point, referred to as the xe2x80x9cfocal spot.xe2x80x9d This target surface is comprised of a refractory metal having a relatively high atomic number so that when the electrons collide with the target surface at the focal spot, a portion of the resulting kinetic energy is converted to electromagnetic waves of very high frequency, i.e., x-rays. The resulting x-rays emanate from the target surface, and are then collimated for penetration into an object, such as an area of a patient""s body, and then used to produce an x-ray image. In many applications, such as a CT system, precise control over the size and shape of the focal spot is critical for ensuring a satisfactory x-ray image.
In general, a very small part of the electrical energy used for accelerating the electrons is converted into x-rays. The remainder of the energy is dissipated as heat in the anode target region and the rest of the anode. This heat can reach extremely high temperatures that can permanently damage the anode structure, and/or can reduce the operating efficiency of the tube. To alleviate this problem, the x-ray target, or focal track, is typically positioned on an annular portion of a rotatable anode disk. Typically, the anode disk (also referred to as the rotary target or the rotary anode) is mounted to a rotor assembly having a supporting shaft that is rotatably supported by bearings contained within a bearing housing. The rotor assembly and disk are then appropriately connected to and rotated by a motor. During operation, the anode is rotated and the focal track is rotated into and out of the path of the impinging electron beam. In this way, the electrons are striking the target at specific focal spots for only short periods of time, thereby allowing the remaining portion of the track to cool during the time that it takes to rotate back into the path of the electron beam. This reduces the amount of heat generated at the target in specific regions, and reduces the occurrence of heat related problems in the anode target.
The rotating anode x-ray tube of this sort is used in a variety of applications, some of which require the anode disk to be rotating at increasingly high speeds. For instance, x-ray tubes used in mammography equipment have typically been operated with anode rotation speeds around 3500 revolutions per minute (rpm). However, the demands of the industry have changed and high-speed machines for CT Scanners and other applications are now being produced that operate at anode rotation speeds of around 10,000 rpm and higher. These higher speeds are necessary to evenly distribute the heat produced by electron beams of ever-increasing power.
The higher operational rotating anode speeds, and the higher heat loads typical of the newer x-ray tubes, contribute to a variety of problems. For instance, much higher stresses are placed on the bearings, and the other portions of the anode drive assembly, due to the forces exerted as a result of the high rotational speeds. These mechanical stresses are exacerbated in the presence of the high operating temperatures of an x-ray tube. Existing drive assemblies have not been entirely satisfactory in dealing with these extreme operating conditions. For example, a typical prior art anode drive assembly is constructed with multiple components having different material types, and which are interconnected with numerous braze and/or weld joints. This use of multiple components, and multiple connection points, are subject to failure, and can be a source of mechanical instability. For example, excessive heat can cause the physical connections in the anode rotor structure and bearing assembly to loosen, especially when the component parts and/or the braze joints are constructed of different metals that have dissimilar coefficients of thermal expansion (CTE). Points of mechanical instability can also arise where interconnected parts have improper mating surfaces, are improperly assembled, and/or have insufficient fastener preloads. Again, each of these problems are further exacerbated in the presence of the extremely high thermal stresses encountered within the rotor assembly. Any one of these problems can contribute to the instability of the rotor assembly, which results in a non-stable rotation of the anode target. This is manifested in unpredictable movement and positioning of the focal spot on the target, which degrades the resulting x-ray image quality.
In addition to diminishing the quality of the x-ray image, any mechanical instability in the anode drive assembly can result in other problems as well. For instance, it can result in increased noise and vibration, which can be unsettling to a patient and distracting to the x-ray machine operator. Also, unchecked vibration can shorten the operating life of the x-ray tube.
In light of the foregoing problems, what is needed is an improved anode drive assembly that can be used to support and rotate a target anode in a x-ray tube. In particular, the drive assembly should permit the anode to be rotated at very high speeds without vibrating or generating noise. Moreover, the drive assembly should maintain this mechanical stability, even in the presence of high operating temperatures.
The present invention has been developed in response to the present state of the art, and in particular, in response to these and other problems and needs that have not been fully or completely solved by currently-available drive assemblies for use in connection with x-ray tubes having rotating anodes. Thus, it is an overall object of the present invention to provide an anode drive assembly that is capable of rotating an anode target at high rotational speeds, and that can do so with minimal vibration and noise. A related object of the invention is to provide an anode drive assembly that maintains mechanical stability even in the presence of high operating temperatures. Further, it is an objective to provide an anode drive assembly that reduces the amount of heat that is conducted from the anode target to more heat sensitive portions of the bearing assembly, such as bearings and bearing surfaces. Another objective is to provide an anode drive assembly that utilizes fewer components and fewer attachment points, thereby reducing the opportunity for mechanical failure due to disparate thermal expansions between components, joint failure, improper component fit, improper assembly, and the like. Also, it is an objective to provide an anode drive assembly that is assembled in a manner so that there is a gradual transition in the coefficient of thermal expansion along the thermal conductive path between the anode and the bearing assembly. This ensures that adjacent components have closely matched coefficients of thermal expansion, thereby reducing mechanical stresses that may result in the presence of high operating temperatures.
In summary, the foregoing and other objects, advantages and features are achieved with an improved rotating anode drive assembly for use in connection with an x-ray tube having a rotating target anode. Embodiments of the present invention are particularly suitable for use in connection with x-ray tubes used in equipment requiring high anode rotational speeds, and in which precise control over the focal spot position is requiredxe2x80x94such as CT scanner x-ray tubes.
In a preferred embodiment, the anode drive assembly is comprised of a target rotor assembly, which is connected to the anode disk via a shaft portion. The target rotor is rotatably supported by a bearing assembly having a bearing shaft that is rotationally supported via a bearing surface. The target rotor preferably provides an inductive motor capability, such that rotating motion can be provided to the anode via the target rotor.
In a preferred embodiment, the bearing assembly is operatively connected to the rotor assembly via a bearing hub. The bearing hub preferably includes means for reducing the amount of heat that is transferred from the anode to the bearing shaft and other portions of the bearing assembly. In one embodiment, this is accomplished by minimizing the conductive heat path between the anode to the rest of the bearing assembly via the structure of the bearing hub.
Preferred embodiments improve the mechanical and thermal attributes of the anode assembly in other ways as well. Preferably, the anode assembly is constructed of materials such that there is an incremental increase in the coefficient of thermal expansion between the target anode and the bearing surfaces of the bearing assembly. This gradual transition in thermal expansion rates reduces the amount of thermal and mechanical stresses that occur along the assembly during operation of the x-ray tube. Moreover, the bearing assembly is preferably constructed so that components immediately adjacent to the anodexe2x80x94namely the rotor shaftxe2x80x94experience substantially the same rate of thermal expansion as the anode itself. These factors all contribute to the overall mechanical stability of the drive assembly, and ensure precise rotation of the anode, accurate and consistent placement of the focal spot, and increased x-ray image resolution. Further, the increase in mechanical stability results in an x-ray tube having less operational vibration, and consequently, that produces less operating noise. Also, lower vibration reduces the incidence of x-ray tube failure.
These and other objects, features and advantages of the present invention will become more fully apparent from the following description and appended claims, or may be learned by the practice of the invention as set forth hereinafter.